1. Technical Field of the Invention
The present invention relates to the protection of sensors from mechanical shock, vibration, and impact, and to the enhancement of a sensor's response by improving its coupling to the environment.
2. Description of the Prior Art
A mechanical isolator for an accelerometer is described in the Sandia National Laboratories technical report number SAND2000-1528, entitled “High Shock, High Frequency Characteristics of a Mechanical Isolator for a Piezoresistive Accelerometer, the ENDEVCO 7270AM6” by Bateman, Brown and Nusser (2000). This device has two metal plates that, when held together by screws, clamp the accelerometer between them. Between each plate and the accelerometer is a thin sheet of a polysulfide elastomer to provide the isolation. The isolator's mass is about 7 grams, compared with 1.5 grams for the accelerometer.
This reference is deficient with respect to the present invention in that it is massive and bulky, and it relies upon an elastomer for the isolation. In some situations, elastomers and other dissipative materials are unsuitable because their stress-strain behavior is non-linear, as well as frequency dependent and temperature dependent. Elastomers may also be unsuitable because they degrade with time, and some react chemically with the environment.
The use of periodically layered media for isolation is described by Sackman, Kelly and Javid in “A layered notch filter for high-frequency dynamic isolation,” Journal of Pressure Vessel Technology, February 1989, Vol. 111, pages 17-24. They present a mathematical analysis of the propagation of elastic waves through a periodic or repeating arrangement of two alternating layers. Experiments demonstrate the validity of the concept.
This reference is deficient with respect to the present invention in that only repeating arrangements of materials are considered.
The company Brüel and Kjaer markets “mechanical filters for accelerometers,” which are described in Brüel and Kjaer product data sheet for parts UA 0553 and UA 0559. These filters are interposed between an accelerometer and the object to which it is mounted in order to prevent the accelerometer from detecting high frequency vibration. The filter is comprised of upper and lower steel parts bonded together with a butyl rubber core. The butyl rubber is the filter medium.
This reference is deficient with respect to the present invention in that it relies upon an elastomer for filtering. The stiffness and damping effect of the butyl rubber depends upon temperature, with optimum damping at ambient temperatures of 20 to 50 degrees Celsius. At higher temperatures the stiffness and damping of the rubber are reduced, while at lower temperatures the stiffness and damping of the rubber are increased.
In U.S. Pat. No. 5,312,675, Cooper et al described an article of protective clothing to protect against blast. The clothing has two sheets of material, chosen so that they have an acoustic decoupling effect which reduces the pressure wave effect on the body of a person wearing the clothing. The design approach is similar to the one used in the current application, in that the garment's layers are chosen specifically so that they will alter the mechanical wave transmitted through them and into the body in a specified manner. The approach is described in greater detail by the authors and a colleague in “The role of stress waves in thoracic visceral injury from blast loading: modification of stress transmission by foams and high-density materials,” Journal of Biomechanics, Volume 24, Number 5, 1991, by G. J. Cooper et al.
This reference is deficient with respect to the present invention in that it is not applicable to the end use application intended for the present invention.